Method and system of carrier-wave telegraphy



Oct. 30, 1928. 1,689,346

- B. W. KENDALL METHOD SYSTEM OF CARRIER WAVE TELEGRAPH! Filed Oct. 31-. 1919 z Sheets-Sheet 1 Q/MWZTAZIQ Oct. 30, 1928.

Filed Oct. 31. 1919 2 Sheets-Sheet 2 HIRE? Patented Cd. 30, 1928.

UNITED STATES PATENT OFFICE.

BURTON W. KENDALL, OF NEW YORK, N, Y., ASSIGNORTO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF YORK, N. Y., A CORPORATION OF NEW YORK.

METHOD AND SYSTEM OF CARRIER-WAVE TELEGRAPHY.

Application filed October 31, 1919. Serial No. 334,682.

This invention relates generally to the distant control of apparatus for signaling or other purposes and particularly to the employment of alternating currents or current 5 waves of different frequencies for effecting said control.

More particularly the invention relates to the control of signaling or other apparatus by waves of different frequencies jointly,

which waves may be transmitted independmultiplex or a simplex system employing high frequency waves may give an indication to the distant transmitting operator while.

signal transmission is in progress.

It 1s customary 1n telegraphy to provlde for breaking in during the transmission of a message in case the receiving operator desires for any reason to have the transmission stopped. At times the receiving operator may fail to get a Word, and also in the case of a mechanical receiver, it may be necessary, due to some faulty action, to have a part of a message repeated. In the wellknown direct current telegraph systems such means have long been provided.

The advantages of having means for sending a break signal in any telegraph system, especially in those equipped for two-way signaling, are apparent. Such means have not, however, been adopted in systems employing high frequency waves either transmitted on wires or radiated through space, on account, probably, of the difliculties involved. These high frequency wave systems, particularly the type employing line wires r for transmission, are usually multiplex sys- .tems having-several high frequency channels through which independent communications can be carried on, and a practical requirement for such a system is that'the break transmitted through one channel shall not disturb another channel of the system.

In the particular embodiment of the invention herein disclosed, means will be described whereby each receiving operator in a carrier wave or high frequency multiplex telegraph system may break in on the sendmg operator onhis particular channel without afiecting the other channels. While the system is spoken of as employmg high frequency waves, the range ofthe frequencies that may actually be used is very wide and may extend from a few hundred cycles per second or less to several thousand cycles for lines, and even. higher for radio systems.

Briefly stated, according to the principle employed in the system herein shown, there is sent through each channel of the multiplex system a wave of constant frequency from one station and a wave of constant but slightly different frequency from the station that is to be communicated with. Each channel is distinguished therefore by a combination of frequencies'rather than by a single frequency. These two waves interact at each station to produce a wave of a frequency which is the difference between their two frequencies. A receiver at each station is operated selectively only by the difference-frequency wave. Since it is necessary for both frequencies to be present at both stations to operate the receivers, it is seen that each operator may interrupt the control of his own and the distant operators receiver by cutting off the supply of the wave from his station. The sending operator receivesthe break indication as a silencing of his.; .so u nder or other receiver, since it no longer responds to the depression of his key. Further objects and features of the invention will appear from the following description taken in connection with the drawings, of which Fig. 1 is a schematic circuit diagram of a multiplex system according to the invention; Fig. 2 shows a modification of the system of Fig. 1, and Fig. 3 shows a modified circuit arrangement. Like parts are indicated by like reference characters throughout the several views.

Referring first to Fig. 1, the multiplex line 1 is shown with four stations 8,, S S and S connected for intercommunication thereover. In the description it will be assumed that sta tions S and S are arranged to communicate with each other and that stations 8, and S communicate with each other, although, of course, the stations might equally well be differently related and the number of stations on a line is not limited to four.

The apparatus at station S is typical of that at the other stations and includes a generator 0 of undamped waves of a definite frequency assigned to that station. The generator may be of any suitable type, but is shown as comprising a thermionic relay 2 having an oscillation circuit 3, which determines the frequency generated, and a connection 4 for feeding back energy from the output side to the input side to control the generation of the Waves.

An amplifier A is shown connected to the generator and. has included in its output cirwit, a circuit 5 tuned to the frequency of the generated waves. From the amplifier the Waves are impressed upon an intermediate circuit 6, also tuned to the frequency of the generated wave, which includes a key 7 shunted by a normally closed switch 8, so that normally the continuous wave is impressed upon the coils 9 and 10 and through the line branch 11 to the line 1. K

At station S a generator which may be of the same type as 0 or any other suitable type, is provided to supply a current Wave,

through the tuned intermediate circuit 12 and the line branch 13, to the line 1. The switch 14 is normally closed to short-circuit key 15.

The frequency of the wave from generator 0 is different from that of generator 0,, so that when both generators are operating and the circuits 6 and 12 are closed, as they normally are, there are two continuous waves on the line 1 having frequencies which may be designated 7, and 7, from generators O and 0 respectively.

Each band filter 16, which consists of selective elements, either inductance of capacity or both, in series with the branch 11 and also in shunt of said branch, as indicated, is designed to transmit a range of frequencies sufficient to include frequencies f, and f but the frequencies outside of this range are excluded from branches 11 and 13 by these filters. For detailed information as to the construction and design of filters of this type, the patent to G. A. Campbell, 1,227,113, issued May 22, 1917, may be consulted.

In the usual carrier current system, the

coils 9 and 10 would be arranged with the network N and with the bridged coil 17 in the Well-known manner, so that the portion of the circuit to the right of the point in the drawing to which coil 17 is tapped, and including the net N, would form a balance for that portion of the circuit lying to the opposite side of said points as to the frequency transmitted from the generator 0 With an exact balance of this sort the coil 17 would be neutral as to currents transmitted from the generator 0 In the present system, however, it is necessary to feed some of the energy from generator 0 into the local receiving circuit, and the particular way of doing this, in the case of the system shown in Fig. 1, is to adjust the net N for a slight unbalance so that with the key 7 or the switch 8 closed, a small portion of the energy from generator 0 is impressed on the coil 17, due to the unbalance.

The receiving circuit, comprising a first thermionic detector 1),, and a second detector D is coupled to coil 17. The wave of the second detector D by which the beat fre quency wave is converted into a more or less continuous current impulse and is thus rendered capable of affecting the relay 20.

Relay 20 may control a local circuit, a portion of which is indicated, in which may be any suitable receiver such as a sounder or recorder in accordance with well-known practice.

In a similar manner, the wave of frequency f, transmitted from generator 0 over the line is passed from branch 13 to the bridged coil 21 and also some of the current of frequency f from generator 0 passes into coil 21 on account of the slight unbalance of the net N at station S and the line, as explained in connection with coils 9 and 10 and 17. These waves are both impressed upon the in put circuit of detector D',. The difference or beat frequency produced in the detector D is selectively passed to the second detector D and is finally rendered capable of actuating relay 24 which may be similar to relay 21.

If it is desired to transmit signals from station S to station 8,, the switch 14, which is normally closed, is left closed; switch 8 is first opened, and the key 7 is actuated in accordance with the signals.

Since, as explained, the relays 20 and 24 are operated by the beat-frequency wave, it is necessary that both waves of frequency f, and f be applied to the receiving circuits at their respective stations in order to energize However, if the operator at station S, wants to break-in on the operator at station S he merely opens switch 14 which cuts off the supply of current waves from generator 0, to both receiving circuits, and neither relay nor 24thereafter responds to the actuation of key 7, so that the sending operator at station S is notified of the receiving operators break-in.

The operator at station S may hold down his key 7 or close switch 8 and the operator at station S can then transmit to station S by actuation of key 15 with the switch 14 open. The manner of sending and receiving so far as the operators are concerned, it will be recognized, is quite similar to that in the well-known closed circuit Morse system.

In additibn to the apparatus shown at station S,, the station S is shown as having tuned circuits 22 and 23. Similar circuits could, of course, be provided at the other sta-, tions. These two circuit, together, are tuned to pass with minimum attenuation two frequencies, f, and f,. This double tuning may be obtained by closely coupling circuits 22 and 23 and otherwise adjusting their constants. By virtue of this double tunin both of the frequencies f, and f, are selective ly impressed upon the detector D but high impedance is offered to other frequencies. This circuit arrangement is particularly useful in simplex circuits where the band filter 16 would not ordinarily be employed.

Stations S and S are provided with apparatus similar to that shown at stations S and S except that generators O and 0 have frequencies which are different from each other and different from those of generators and O and the band filters 25 are arranged to pass the frequencies generated by generators O and(),. Stations S and S therefore communicate with each other through a different high-frequency channel from that used by stations S and S Transmission between stations S and S is accomplished similarly to that between sta tions S and S With circuit 27 closed and circuit 26 intermittently closed, the relays 28 and 29 both respond and when either circuit 26 or 27 is open neither relay can be energized by the closing of the other circuit. Between the dotted lines, in the spaces 30 and 31, there may be assumed to be the necessary apparatus such as detectors D and D and if desired the circuits 22 and 23.

In the circuit of Fig. 2 there is shown a modified arrangement for feeding energy from the generator 0 (which to simplify the drawing is shown instead of generator 0 at station S to the circuit of the local detector D,. The detector circuit is coupled to the bridged coil 17 through tuned circuits 32 and 33 similar in action to circuits 22 and 23, that is, they are arranged to be resonant to the two frequencies f, and f Circuit 6 has a coupling with circuit 32 indicated at 34, so that when key 7 or switch 8 is closed the output wave for generator 0 is fed both to the line 1 and to the circuit 32. In this case it is not necessary to unbalance the line and net. Coupling 34 may be adjusted to regulate the supply of energy into the circuit 32.

Referring now to Fig. 3 the main line 1 is provided with a balancing net N, as shown, for all the frequencies that are to be transmit ted on the line. By means of the usual line coils and 41 and the output coils 42, waves of all frequencies transmitted to distant stations pass into the branch 44 and waves to be received from the line are fed into the branch 43.

Two stations S and S are shown connected to the line through the branches 43 and 44 which may be. arranged to communicate respectively with distant stations S and S not shown in this figure. Incoming waves over line 1 from station S pass through branch 43 and are selected by the circuit 45, tuned to the frequency 7, from station S,, from which circuit they are fed into circuit 46. i

Generator 0, which may be of the same type as generator 0 of Fig. 1 or any other suitable type, is coupled to circuit 47 tuned to the generator frequency f,, and circuit-47 is coupled in turn to the tuned circuit 48 connected across branch 44 so that with circuit 47 closed, the wave from generator 0 is fed to the line 1.

Circuit 47 also has a coupling with circuit 46 so that when the circuit 47 is closed, some of the current from generator 0 is impressed on circuit 46 together with current from sta tion S as explained. The band filter 49 is capable of passing both frequencies f, and f, to the detector circuit 50 which may contain elements identical with those shown in the receiving side of either station S or station S. of Fig. 1, the effect of this arrangement being that relay 51 responds only to both current waves from the stations S and S,,.

In the alternative arrangement shown at station S ,.of Fig. 3, the circuits 53 and 54 are tuned to the two frequencies which are to be impressed on the receiving circuit of station S, for reasons explained heretofore. Circuit 52is permanently closed and the key is placed in the generator circuit so that when the key is closed energy is fed to both circuits 52 and 53. The coupling between circuit 52 and 53 is only through the generator circuit and may .be very small.

The same method of transferring energy from the generator to the local receiving circuit could, of course, be employed at both the stations S and S of Fig. 3. The stations S and S with which the two stations of Fig. 3 communicate may be associated with line 1 in the same manner as are the stations in Fig. 3, or the stations S and S of Fig. 1 may be assumed to be connected to the line 1 of Fig. 3.

While the invention has been described with reference to signaling it is not limited to signaling but is equally applicable to the con trol of apparatus by waves of different frequencies whether the waves be in the form of currents on a transmission line or waves radiated through space. The invention is not to be limited to the particular type of receiving circuits or generators shown nor is the invention to be otherwise limited by the particular circuit arrangement as a whole or to the specific details, but only by the scope of the appended cla ims,

What is claimed is:

1. In a multiplex system, stations connected by a line, a network for substantially balancing said line for signal frequencies supplied thereto, means for transmitting current waves of different constant frequencies from each of said stations to said line, at

each of said stations a plurality of signal receiving means each supplied wit-h .a wave out going from said station due to unbalance, and means for selecting combinations of two of said waves to actuate each of said receivers,

2. In a signaling system, stations, a plurality of means at each of said stations each transmitting a wave, of frequency diiferent from the others, means at each of said sta tions for separately receiving the waves of different frequencies transmitted to said station, a receiver for each said separately received wave, means for supplying to each said receiver a wave from one of said transmittin means, each of said receivers being actuated by the conjoint action of the separately received wave and said transmitted wave.

3. A signaling system comprising a line having stations thereon, a balancing network for said line, a plurality of communication channels for said line, each channel being traversed by a current of constant frequency transmitted from one station and by a current of different constant frequency transmited from another station, and a receiver for each of said channels at each of said stations selectively supplied with transmitted current and actuated by the conjoint action of said tvfi currents traversing said channel, whereby a number of two-way transmissions may be accomplished by using twice the number of waves of different frequencies.

4. In a signaling system, stations, a line between said stations, means for balancing said lines, means for supplying current of a given frequency interrupted in accordance With signals to said line at one station, a receiver for said signals at each station, a source of current of a different frequency at one station, connections for associating said source with said line and with the receiver at said station, and means whereby said signals are registered in said receivers only while said connections are made. I

5. A line for the transmission of a plurality of current waves of different frequencies, stations for said line, a net for balancing said line for the transmitted frequencies, a detecting circuit for said line, a source of waves of constant frequency at each of said stations, means for simultaneously applying a wave interrupted in accordance with slgnals from one said source to said line to said detecting circuit and a continuous wave of difi'erent but constant frequency from said line tosaid detecting circuit, and a receiver controlled by said detecting circuit.

6. A line for the transmission of a plurality of current waves of difl'erent frequencies, stations for said line, a net for balancing said line for the transmitted frequencies, a detecting circuit for said line at each station, a source of continuous current waves of constant but different frequency at each station, means for simultaneously applying waves from each of said sources to said line and to one of said detecting circuits and from said -line to each of said detecting circuits, and a receiver associated with each detecting circuit actuated by 'the conjoint action of a wave from said line and a wave from said source.

7. The method which comprises transmitting one wave, receiving another wave, locally producing a third wave having an amplitude proportional to the products of the first and second mentioned waves, selecting a component of the third wave, and convertin by detection the wave form of the selected component.

In, witness whereof, I hereunto subscribe my name this thirteenth day of October A. D., 1919.

BURTON W. KENDALL. 

